Research On Friction Of Three-dimensional Drill String In Full Well

Drill string mechanics,as an important research content in oil drilling engineering,provides theoretical research foundation for the problems of drill string under the force,drilling direction control,buckling instability and vibration damage.Many scholars have devoted themselves to this research work and have achieved many achievements.They mathematically abstract the drill string from different downhole conditions and use different calculation methods to calculate the stress state of the drill string,forming some typical drill string mechanical models and solution methods.These models and methods have solved some problems in the field project,but each has certain limitations.Few mechanical models can be applied to the drilling string mechanical calculation of the whole wellbore.However,the frictional resistance of the drill string is not only caused by the resistance of a certain part,but of the entire drill string.Based on the previous research,this paper proposes a method for quickly and efficiently calculating the force of the drill string in the entire wellbore on statics.Contact force has always been a trouble in the mechanical analysis of drill string,because the contact distribution of the drill string to the well wall cannot be known in advance in the axial direction of the wellbore in the three-dimensional wellbore,and the contact angle in the cross-sectional direction of the wellbore cannot be judged in advance as well.Due to the uncertainty of the state,the contact force and other loads cannot be accurately calculated by the formula.The traditional treatment method is based on the bending theory of the soft rope.It is judged whether the drill string will exhibit sinusoidal buckling or spiral buckling according to whether the axial force exceeds the buckling critical force,and it is assumed that the contact area will maintain continuous contact.Then the contact force can be calculated by determined contact pattern.As the scholars continue to study and find that the analysis results based on the soft-column model have a large deviation from the actual,thus the rigid-column model is proposed.And the assumption of contact area is no longer continuous contact,but point contact,which would be verified by a corresponding analogy experiment in this paper.In this paper,the idea of the flexibility matrix of the predecessors is used in the calculation of the contact force: the contact force is calculated by the deformation displacement of the drill string.The flexibility coefficient matrix is calculated by finite difference model.And the model is established by the basic mechanical equations which include the equilibrium differential equation of the drill string and the elastic differential equation.Based on the previously established flexibility matrix equation,the particle swarm optimization algorithm is introduced,and some contact angle positions are randomly initialized according to the determined contact distribution.Once the contact position is determined,the whole state of the drill string is determined,whereby the deformation displacement becomes known,and the contact force can be calculated by the flexibility matrix equation.Only the contact force calculated at this time needs to be judged according to theory and practice,and the particle swarm algorithm is to ensure that the contact position of each search is as close as possible to the actual situation.Although the particle swarm optimization algorithm greatly simplifies the solution of the contact angle position,it still does not solve the pre-determined problem of contact distribution.Therefore,this paper proposes a "Relaxation Search Algorithm" to search all possible contact sets quickly by gradually relaxing the contact points.Since the time complexity of this algorithm is proportional to the square of the total number of drill string nodes,it will be time-consuming in long-distance calculation.Therefore,the parallel computing of image processor(GPU)is added according to the parallelism of particle swarm optimization algorithm.In the calculation of large data volume,its speed far exceeds the parallel computing of CPU,which makes it possible to apply the calculation method in the calculation of drill string force in whole borehole.In the analogy experiment and algorithm research and analysis of this paper,the following conclusions are drawn:1)The contact assumption between drill string and borehole wall in traditional soft-string model and rigid-string model adopted by some scholars is continuous contact,but he problem is that the experimental results show that the downhole drill string does not present continuous contact between the well walls,but is in point contact.The experiment shows that the pipe string with residual bending will increase the friction resistance during drilling,the friction resistance has been calculated according to the size of the actual working condition is about 2.3 times that directly estimated by gravity;2)Compared with FMDS algorithm,this paper optimizes the layout of flexibility matrix,simplifies the program logic and speeds up the algorithm.The processing of flexibility matrix indirectly speeds up the overall calculation speed;3)This paper directly searches the possible contact location by particle swarm optimization algorithm,optimizes the iteration process of ABIS algorithm and FMDS algorithm,and maximizes the use of the idea of flexibility matrix;4)The relaxation search algorithm proposed in this paper combined with particle swarm optimization algorithm can search all the accessibility of drill string in the borehole globally.The energy state solution and GPU parallel computing are introduced to speed up the whole search algorithm.